Potential Impact of Global Climate Change on Species Richness of Long-Distance Migrants

Little evidence exists demonstrating that global climate change leads to systematic changes in the structure of ecological communities. For avian communities, one would expect warmer winters to lead to declines in numbers of long-distance migrants if resident birds benefit from warmer winters and impose increasing competitive pressure on migrants. To study the potential influence of global climate change on long-distance migrants, we correlated the number of all species of land birds and the number and proportion of long-distance migrants, shortdistance migrants, and residents in 595 grid cells across Europe. We used mean temperature of the coldest month, mean spring temperature, and spring precipitation as measures of climatic conditions in winter and during the breeding period. The number and proportion of long-distance migrants decreased with increasing winter temperature, decreasing spring temperature, and increasing spring precipitation. We used this spatial relationship between bird community structure and climate in Europe to make predictions about changes in bird communities of the Lake Constance region, Central Europe, between two census periods (1980–1981 and 1990–1992). Winter temperature in this region increased significantly between the two censuses, whereas spring temperature and precipitation did not change. As predicted from the models, the proportion of long-distance migrants decreased and the number and proportion of short-distance migrants and residents increased between the two censuses. The significant declines of the long-distance migrants in the Lake Constance region are of a magnitude that can be explained by the observed climate change. Our results suggest that increasingly warmer winters may pose a more severe threat to long-distance migrants than to the other bird groups. Impacto Potencial del Cambio Climático Global Sobre la Riqueza de Especies Migratorias de Larga Distancia Resumen: Existe poca evidencia que demuestre que el cambio climático global conduce a cambios sistemáticos en la estructura de las comunidades ecológicas. Para las comunidades de aves, se podría esperar que inviernos más cálidos conducirían a declinaciones en los migrantes de larga distancia si las aves residentes se benefician de los inviernos más cálidos e imponen una presión competitiva creciente sobre las migrantes. Para estudiar la potencial influencia del cambio climático global sobre migrantes de larga distancia, correlacionamos el número total de especies de aves terrestres y el número y la proporción de migrantes de larga y corta distancia y los residentes de 595 celdas en Europa. Utilizamos la temperatura promedio del mes más frío, la temperatura promedio de la primavera y la precipitación primaveral como medidas de las condiciones climáticas en invierno y durante el período reproductivo. El número y proporción de migrantes de larga distancia declinó con el aumento de la temperatura invernal, la disminución de la temperatura primaveral y el aumento de la precipitación primaveral. Utilizamos esta relación espacial entre la estructura de la comunidad de aves y el clima en Europa para predecir los cambios en las comunidades de aves en la región del Lago Constancia, Europa Central, entre dos períodos censales (1980–1981 y 1990–1992). La temperatura invernal en esta región aumentó significativamente entre los dos censos, mientras que la temperatura y precipitación primaveral no cambiaron. Tal como predijeron los modelos, la proporción de migrantes de larga distancia disminuyó y el número y proporción de migrantes de corta distancia y residentes aumentó entre los dos censos. Las declinaciones significativas en los migrantes de larga distancia en la región del Lago Constancia son de tal magnitud que se pueden explicar por el cambio climático observado. Nuestros resultados sugieren que los inviernos cada vez más cálidos posiblemente sean una amenaza más severa para migrantes de larga distancia que para otros grupos de aves. * email [email protected] Paper submitted August 8, 2001; revised manuscript accepted June 12, 2002. 578 Climate Change and Long-Distance Migrants Lemoine & Böhning-Gaese Conservation Biology Volume 17, No. 2, April 2003 Introduction Global climate change is currently of great scientific interest (Bawa & Markham 1995; Dai et al. 1997; Hulme et al. 1999). Earth surface temperature has increased since 1860 by 0.5 to 0.7 C (Cao & Woodward 1998). One possible reason for this phenomenon is the accumulation of CO 2 and other greenhouse gases in the atmosphere ( Graham & Grimm 1990 ). Scientists predict a much higher increase of Earth surface temperature for the twenty-first century (Houghton et al. 1992; Bengtsson 1997). Increases might reach 5.8 C in the next 100 years (McCarthy et al. 2001). Recent climate change has already had a significant influence on Earth’s flora and fauna. In Europe, for example, the growing season of plants has become longer (Menzel & Fabian 1999). In North America, climate change has shortened the hibernation period of a marmot species (Inouye et al. 2000). For birds, evidence exists of the influence of climate change on phenological patterns (Alerstam 1990; Chesser 1998 ). Changes in climate have led to many bird species extending their geographic ranges northward (Thomas & Lennon 1999). Some migratory birds arrive earlier on their breeding grounds (Mason 1995), start breeding earlier (Winkel 1997; Crick et al. 1998; Forchhammer et al. 1998; McCleery & Perrins 1998; Crick & Sparks 1999; Dunn & Winkler 1999), or migrate to their wintering grounds later (Gatter 1992; Bezzel & Jetz 1995). However, little evidence exists that climate change leads to systematic changes in species richness and the structure of bird communities. Nevertheless, theoretical considerations predict a change in the relative abundance of migratory and resident birds. Increasing winter temperatures may cause a decline in the number of migrants. In springtime, migrants arrive on their breeding grounds and share existing resources with overwintering residents (MacArthur 1959; Rabenold 1979, 1993). During less severe winter conditions the survival rate of residents is probably higher, which may lead to larger numbers of residents surviving the winter. If diffuse competition exists between migratory birds and residents, this should lead to fewer resources being available to migrants (Herrera 1978). As a result, the breeding success and abundance of migrants should decline. This hypothesis is supported by O’Connor (1990), who found a decrease in the abundance of residents after severe winters and an increase in the number of migrants the following spring. Similarly, Newton and Dale (1996 a , 1996 b ) and Willson (1976) have demonstrated that the proportion of migrants increases with increasing latitude (i.e., decreasing mean winter temperature). Even long-term trends in bird populations may be caused by climate change. An analysis of the bird-atlas data of the Lake Constance region in Central Europe between 1980–1981 and 1990–1992 showed significant declines in long-distance migrants, whereas short-distance migrants and residents increased slightly (Schuster et al. 1983; Bauer & Heine 1992; Böhning-Gaese & Bauer 1996). This data set shows one of the strongest declines in long-distance migrants recognized so far. One cause for these declines may be warmer winter temperatures (Böhning-Gaese & Bauer 1996), but there is no evidence to support this theory. To understand the relationship between the proportion of migratory and resident birds and climate factors within a region, we studied the spatial variation of bird communities and climate in Europe. Our objectives were, first, to examine the spatial pattern of the number and proportion of migratory and resident bird species in Europe. Second, we wanted to quantify the relationship between bird community structure and climate factors in Europe. Third, we sought to predict population changes in the Lake Constance bird communities between 1980–1981 and 1990– 1992 by examining the relationship between the structure of European bird communities and climate factors. Using the existing data of the Lake Constance Bird Atlas (Schuster et al. 1983; Bauer & Heine 1992; Böhning-Gaese & Bauer 1996), we tested whether the changes in the structure of the bird communities could have been caused by climate change.